Various contrivances such as, for example, carrying out opening/closing operation smoothly and the like, for back doors are known (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 2007-245747, JP-A No. 9-220934, JP-A No. 2003-291645, JP-A No. 11-350827, and JP-A No. 8-324255).